In recent years, the demand for high definition and low power consumption in a light emitting display device for use in mobile devices has increased. Display devices which use liquid crystal display devices (LCD) or self-light emitting elements such as organic EL elements (organic light-emitting diode: OLED) or quantum dots in a display part or electronic paper and the like are being adopted as display devices for use in mobile devices.
A display device using a self-light emitting element such as that described above does not require a backlight light source or polarization plate that are require in liquid crystal display devices and since the drive voltage of a light emitting element which is a light source is low, such devices are attracting attention as low power consumption and thin light emitting display devices. In addition, because it is possible to form a display device with only a thin film, it is possible to realize a flexible display device. Furthermore, since a glass substrate is not used, such devices are attracting attention for being light and difficult to break.
In particular, in an organic EL display device, light is output by flowing a current to a light emitting element. Although the light which is output from a light emitting element proceeds to the side of an observer via other layers, a part of the light is reflected at a boundary where different layers contact, the light is wave-guided through the layers, does not reach an observer and cannot be used effectively. In addition, the wave-guided light is transmitted to a pixel adjacent to a pixel which attempts to emit light and is output to the exterior from the adjacent pixel which is the cause of what is called leaking light. In particular, in high definition display devices which exceed 300 ppi, around 80 percent of the light which is produced by a light emitting element is lost by being wave-guided through each layer as described above.
Thus, light extraction technology is being developed in order to efficiently extract light output from a light emitting element to the exterior. For example, in Japanese Laid Open Patent Application No. 2012-226931, an extraction structure is proposed in which a high refractive index transparent layer having a higher refractive index than an organic compound which forms a light emitting element is provided on the light extraction side of the light emitting element and a fine concave/convex structure is provided on the light extraction side of the high refractive index transparent layer. According to the Japanese Laid Open Patent Application No. 2012-226931, since a part of the light which is trapped within the light emitting element can not be used effectively and is transmitted to a high refractive index layer and is extracted to the exterior by the light extraction structure, it is possible to realize a brighter display device.
However, in the display device disclosed in Japanese Laid Open Patent 2012-226931, it is necessary to provide the structure described above a light emitting element which leads to a longer processing, accompanying increase in costs and a drop in yield. In addition, in the display device shown in the Japanese Laid Open Patent Application No. 2012-226931, because the light wave-guided in a planar direction of a substrate from a light emitting element is absorbed by a visible light absorbing material, the display device does not contribute to an improvement in a light extraction ratio.